The human eye has two major refracting elements responsible for focusing light on the retina The cornea is the anterior refracting surface responsible for the majority of the focusing power of the eye equivalent to approximately 43 diopters whilst the remaining power of approximately 19 diopters is from the crystalline lens which is a transparent structure that focuses light in the human eye.
Emmetropia refers to a state of focus of the human eye where objects viewed at a distance are in focus on the retina. If axial length of the eye is such that the focal point is in front of the retina the refractive error is known as myopia and if the focal point is beyond the retina the refractive error is termed hypermetropia If the focus of the eye is unequal in different meridians the refractive error is known as astigmatism that can be associated with myopia or hypermetropia.
The power of the cornea is a fixed quantity and the ability to maintain focus for objects closer than infinity depends on altering the focus of the crystalline lens.
The human lens is attached to the wall of the eye by fine filamentary fibers known as zonules that attach the equatorial region of the lens to the region of the eye wall surrounded by a circular muscle known as the ciliary muscle. In an emmetropic individual, when an object in the distance is viewed the ciliary muscle is relaxed and the zonules are under tension. The elastic capsule of the lens is taut and the curvature of the lens is influenced and slightly flattened by these forces. If the object of regard is changed to a near object a reflex stimulates contraction of the ciliary body which relaxes the zonular attachments to the lens. This allows the elastic capsule of the lens and the malleable lens fibers to assume a more spherical shape and due to the increase in pressure in the posterior segment the lens moves forward. Both factors increase the focusing power of the lens by the required amount so that the near object of regard remains in focus. This process is referred to as accommodation.
Up to the 4th decade the human crystalline lens is capable of altering its focus to maintain clarity of vision for near objects by the process of accommodation but this power is gradually lost over the next two decades. This is thought to occur due to several factors including an increased rigidity of the lens fibers, an increase in the diameter of the lens, and possible reduced elasticity of the capsule and zonules. The refractive error describing this inability to achieve near focus is known as presbyopia and is the reason for the progressive need for reading glasses for individuals from the 5th decade of life.
Spectacles or contact lenses placed in the optical pathway can correct the focus for clarity of vision in patients with myopia, hypermetropia, or astigmatism. The correction of presbyopia includes separate glasses for reading, bifocal or multifocal spectacles. Simultaneous near and distant vision can be achieved by multifocal contact lenses that provide simultaneous foci for near and distance. This method however is associated with reduced contrast in vision that may be disturbing to many patients.
More recently it has been found that laser surgery can alter the corneal curvature to correct refractive errors such as myopia, hypermetropia or astigmatism. Phakic intraocular lenses can be placed in the anterior chamber of the eye or behind the iris to correct myopia or hyperopia and the lens can be removed and replaced by an intraocular lens implant for the purpose of correcting a refractive error.
Surgical procedures have also been proposed to restore accommodation in the presbyopic age group. These include sclera implants to expand the diameter of the globe to counteract the expansion of the crystalline lens that occurs with age. Radial incisions at the limbus have also been considered for the same purpose. These procedures however have not proved to be sufficiently reliable or predictable. Laser procedures can alter the corneal curvature to produce a multifocal refracting surface. Corneal implants placed in the corneal stroma can also produce a similar bifocal or multifocal refraction. Finally intraocular lenses with a diffractive or refractive surface can be constructed to provide multiple foci and allow simultaneous focus for distance and near vision. These lenses can be implanted after surgical removal of the crystalline lens or placed in front of the crystalline lens as a phakic implant. Unfortunately all surgical procedures which depend on simultaneous near and distance vision are compromised by reduced contrast and are often associated with undesirable effects such as haloes around lights which may be disturbing to individual patients. The development of an intraocular lens capable of accommodation similar to the crystalline lens in a young individual is therefore extremely desirable.
Opacification of the lens known as cataract formation is a common cause of poor vision in the elderly and can be corrected surgically. Modern cataract surgery is performed by manual extracapsular cataract extraction or by phacoemulsification. Manual extracapsular cataract extraction involves expressing the hard nucleus of the cataract through a 10 mm to 12 mm incision. Phacoemulsification utilises ultrasonic energy transmitted by a needle to fragment the nucleus and allow aspiration of the cataract through a 2.5 mm to 3.2 mm incision. A small incision is desirable in cataract surgery to avoid distortion of the corneal curvature known as astigmatism. In both operations an opening is made in the anterior capsule to allow removal of the lens contents. The capsular bag remnant, however, is left in situ to provide support for an intraocular lens implant which is inserted following removal of the cataract to replace the focusing power of the natural crystalline lens. It is known to provide an intraocular lens implant to replace the cataractous or clear crystalline lens. The power of the lens can be accurately selected prior to surgery so that the patient is emmetropic i.e. clear focus is achieved for objects in the distance. An intraocular lens implant typically comprises a centre focusing element, known as the optic and a peripheral support structure known as the haptic. The haptic of an intraocular lens is the outwardly extending supporting element which interacts with the anterior and posterior leaflets of the capsular bag remnant to ensure fixation and stability. The optic and the haptic of the intraocular lens may be manufactured from transparent rigid plastics material such as polymethyl methacrylate or from flexible plastic materials such as acrylic, silicone or hydrogel polymers. Intraocular lens implants manufactured from flexible materials are preferable to those made of rigid materials because the lens may be folded to allow insertion through a small incision in the sclera or outercoat of the eye and is then allowed to unfold to its original dimensions.
The optic and haptic of the intraocular lens may be manufactured from the same material as a single piece unit or the haptic may be attached to the optic by a variety of mechanisms. There may be one or a plurality of haptics attached to the optic, although the most common configuration includes an optic with two outwardly extending diametrically opposed haptics. The purpose of the haptic is to provide optimal centration of the optic as well as a means of fixation of the implant within a capsular bag remnant of the original lens following cataract or lens extraction. It is preferable that the haptics conform to the periphery of the capsular bag to provide a larger surface area of contact between the intraocular lens implant and the capsular bag and to ensure centration of the optic.
It is also possible to implant a lens in front of the anterior capsule behind the iris with the haptics resting in the region between the root of the iris and ciliary processes, known as the ciliary sulcus. As previously mentioned intraocular lenses may also be inserted in phakic eyes to correct refractive errors, such as myopia or hyperopia. In these circumstances the intraocular lens implant may be placed in front of the crystalline lens behind the iris with the haptic providing support in the ciliary sulcus. Furthermore, as an alternative site of implantation in phakic eyes, intraocular lenses may be inserted in front of the iris in the anterior chamber with the haptics resting in the angle of the anterior chamber.
In all these instances it is preferable that the haptics conform to the periphery of the capsular bag or to the ciliary sulcus or the angle of the anterior chamber in the phakic eye The prior art discloses several haptic designs, including a flange style or loop style, which seek to maximise the surface areas of contact between the intraocular lens implant and the capsular bag. The most common design includes two loop style haptics attached at diametrically opposed points of an optic wherein terminal ends of the haptics extend arcuately towards the periphery of the capsular bag.
The fixation and stability of the intraocular lens implant is not solely dependent on the rigidity of the supporting haptics of an intraocular lens, but is also dependent on fusion of leaflets of anterior and posterior capsule in the interval between the optic of the implant and the terminal of the haptic in contact with the periphery of the capsular bag. It is preferable to maintain as large an interval as possible to provide maximum opportunity for fusion to occur.
Post-operative shrinkage of the capsular bag is not an unusual occurrence. The aforementioned interval may be maintained by a rigid haptic which resists shrinkage of the capsular bag, or by a design for haptics manufactured from flexible plastics which maintains an interval between the terminal of the haptic and the optic in the even of shrinkage of the capsular bag. In order that the design should accommodate the various sizes of capsular bag that will be encountered in different individuals as well as the varying degrees of shrinkage that would occur during the post-operative phase, it is preferable that the haptics should be compressible.
A distinct disadvantage however, of the current haptic designs is that the haptic terminal may be flexed at any point between the haptic terminal and the haptic optic junction towards the optic such that the interval between the haptic terminal and the optic is reduced to the extent where migratory fusion of the leaflets of the anterior and posterior capsule fails to occur. The Author has described (International Publication Number WO00/01323) a haptic design which maintains an interval between the terminal haptic and the optic and provides better conformity of the terminal portion of the haptic with the periphery of the capsular bag.
Previous descriptions of intraocular lenses capable of an accommodative effect include lenses constructed to have an induced increase in curvature of the optic of the lens or a change in position of the lens during attempted focus for near objects. The latter include lenses with a haptic constructed with a hinge to allow forward translation of the optic with attempted accommodation. This occurs due to the increase in pressure in the vitreous or liquid in the posterior segment of the globe induced by contraction of the ciliary body. The accommodative effect of this type of intraocular lens however varies widely in individual patients. The unpredictable results of a intraocular lens with a hinged haptic are due to the differences in fixation that occur with an implant placed in the capsular bag after cataract surgery or removal of the normal crystalline lens by similar techniques. The amount of overlap of the anterior capsular bag leaflets over the haptic and edge of the optic is variable as is the extent and area of fusion of the anterior capsular leaflets to the posterior capsule. This variability interferes with the ability of the hinged haptic to allow forward movement of the optic in response to contraction of the ciliary body in attempted accommodation and is an important factor in the unpredictable accommodative effect that has been encountered in present day accommodative intraocular implants.
The present invention attempts to overcome at least in part some of the aforementioned disadvantages.